Clipping Noise Mitigation in Optical OFDM Systems
Abstract
This letter describes a new non-linear algorithm for clipping noise mitigation in intensity modulation/direct detection dc biased optical orthogonal frequency division multiplexing (DCO-OFDM) systems. Clipping noise is often the major limitation in DCO-OFDM. In this letter, we show that extra information about the clipped signal can be extracted using a non-linear process and then used to mitigate the clipping noise. The effectiveness of the new algorithm is demonstrated by simulation and in an optical wireless experiment.
Index Terms- Intensity modulation/direct detection (IM/DD), optical OFDM, clipping noise.
Objective
OFDM is the main method in the 4G technology. In the optical OFDM system the clipping noise is removed by using a non-linear process.
1. Introduction:
Arthogonal frequency division multiplex- ing (OFDM) is increasingly being considered as a modulation technique for intensity modulated/direct detection (IM/DD) optical wireless communication (OWC) systems. In IM/DD systems, the transmitted signals are modulated onto the intensity of the light and thus have to be unipolar. To make a conventional bipolar OFDM signal into a unipolar signal, one popular approach is to add a dc bias and then clip the OFDM signal at zero. However, for low bias levels, clipping introduces significant distortion which can lead to data detection errors. In this letter we describe a new algorithm for mitigating clipping noise in OFDM and show both experimentally and by simulation that it can significantly improve performance in an IM/DD system. In the algorithm the data is detected and used to regenerate an equivalent time domain signal which is then used to estimate the component of the signal removed by clipping. This is combined with the original received signal and then input into a conventional OFDM receiver. A closely related algorithm was described in for radio frequency (RF) systems, but in the compensation for constellation shrinkage is not optimal and this significantly reduces the effectiveness of clipping mitigation.
Proposed method
Fig. 1. The structure of transmitter and receiver.
4. SOFTWARE AND HARDWARE REQUIREMENTS
Operating system : Windows XP/7.
Coding Language: MATLAB
Tool:MATLAB R 2012
SYSTEM REQUIREMENTS:
HARDWARE REQUIREMENTS:
System: Pentium IV 2.4 GHz.
Hard Disk : 40 GB.
Floppy Drive: 1.44 Mb.
Monitor: 15 VGA Colour.
Mouse: Logitech.
Ram: 512 Mb.
5. Conclusion:
In this letter, a new receiver-based algorithm is presented which applies a non-linear technique to mitigate the effect of clipping in optical OFDM systems. The algorithm uses signal reconstruction to estimate the signal at the transmitter before clipping, and it can be implemented for DCO-OFDM signals with both single-sided clipping and double-side clipping. Simulation and experimental results show that the new algorithm can significantly reduce the BER.
6. References:
[1] J. Armstrong, “OFDM for optical communications,” J. Lightw. Technol., vol. 27, no. 3, pp. 189–204, Feb. 1, 2009.
[2] O. Gonzalez, R. Perez-Jimenez, S. Rodriguez, J. Rabadan, and A. Ayala, “OFDM over indoor wireless optical channel,” IEEE Proc.-Optoelectron., vol. 152, no. 4, pp. 199–204, Aug. 2005.
[3] S. D. Dissanayake and J. Armstrong, “Comparison of ACO-OFDM, DCO-OFDM and ADO-OFDM in IM/DD systems,” J. Lightw. Technol., vol. 31, no. 7, pp. 1063–1072, Apr. 1, 2013.
[4] D. Kim and G. L. Stübler, “Clipping noise mitigation for OFDM by decision-aided reconstruction,” IEEE Commun. Lett., vol. 3, no. 1, pp. 4–6, Jan. 1999.
[5] J. J. Bussgang, “Crosscorrelation functions of amplitude-distorted Gaussian signals,” Res. Lab. Electron., Massachusetts Inst. Technol., Cambridge, MA, USA, Tech. Rep., Mar. 1952.